High tenacity tire yarn

ABSTRACT

THE PERFORMANCE CHARACTERISTICS OF POLYAMIDE AND POLYESTER YARNS WHICH ARE TO BE USED FOR TIRE CORDS ARE IMPROVED BY A METHOD IN WHICH DRAWN LOW TWIST CONTINUOUS FILAMENT YARNS ARE STRETCHED AT A CONSTANT TENSION BETWEEN 75 AND 98% OF THE TENSION REQUIRED TO BREAK THE YARN WHILE EXPOSED TO A TEMPERATURE BETWEEN 200*C. TO 260* C. FOR A PERIOD OF TIME OF FROM ABOUT 5 TO 120 SECONDS. THE YARN AFTER COOLING AT THE STRETCHING TENSION IS CHAR-   ACTERIZED BY HAVING FILAMENTS WHICH ARE ESSENTIALLY ROUND IN CROSS SECTION THROUGHOUT THEIR LENGTH. THE FILAMENTS HAVE CRYSTALLINE DOMAINS HAVING AN ORIENTATION ANGLE LESS THAN 10 DEGREES AND AN X-RAY LONG-PERIOD INTENSITY OF LESS THAN ABOUT 0.4 OPTICAL DENSITY UNITS FOR POLYESTER YARNS AND LESS THAN ABOUT 0.2 OPTICAL DENSITY UNITS FOR POLYAMIDE YARNS.

Fb. 23, 1971 R RQY KEEFE, JR" ET AL 3,564,835

HIYGH TENACITY TIRE mm Filed March 12, 1969 '3 Sheets-Sheet 1 T ENACITY ORA IS /OENIER Feb. 23, 1971 R. LE ROY KEEFE, 4a.. ET AL HIGH TENACITY "mm YARN.

Filed March 12, 1969 IOSODE'NIER NYLON AN-TIOXIOANT 0.02% CuAc 0.2% Kl 10 S EO EXPOSUR E TO HEA TEO N2 3 Sheets-Sheet 2 A 2l5.6 '0 G 237.0 '0

9s 92 BREAK TENSION I2 35 a DJ g u- U) x g v I IOSO-DENIER man 99 g ANTIOXIOANT'F ;o.o2% OuAc 0.2%KI j: 92% BREAK TENSION 30 sec. EXPOSURE TO HEATED n 9 l l l l l .1 a 200 .m 220 I 250 240 250 290 2-10 290 so TEHR'O A-2|s.s2c F|G.4 f- @--291.9c 33 -251.2c 510- 2 G 360- 1' "=2 9 5 IOSO-DENIER mon 96 ANTIOXIDANT 0.02%- OuAc +02% m r 92% BREAK TENSiON mo 2 40 HEATED m '1 INVENTORS so nus sscouns ROBERI LEROY KEEFE JR. WILLIAM OSBORNE STATION ATT RNEY United States Patent Int. Cl. D02g 3/48 US. Cl. 57-140 Claims ABSTRACT OF THE DISCLOSURE The performance characteristics of polyamide and polyester yarns which are to be used for tire cords are improved by a method in which drawn low twist continuous filament yarns are stretched at a constant tension between 75 and 98% of the tension required to break the yarn while exposed to a temperature between 200 C. to 260 C. for a period of time of from about 5 to 120 seconds. The yarn after cooling at the stretching tension is characterized by having filaments which are essentially round in cross section throughout their length. The filaments have crystalline domains having an orientation angle less than degrees and an X-ray long-period intensity of less than about 0.4 optical density units for polyester yarns and less than about 0.2 optical density units for polyamide yarns.

CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 728,125 filed May 10, 1968 and now abandoned.

BACKGROUND OF THE INVENTION In the preparation of cords for tire reinforcement, it is customary to use highly drawn, high-tenacity multifilament polyester or polyamide yarn which is twisted and plied to cord, dipped, and hot-stretched to reduce extensibility of the cord under load and to increase the strength of the cords. Yarns used to make these cords are usually drawn at elevated temperatures, and the resultant cords are also drawn to obtain desirable high tenacities. The corded yarns have many filament crossings and during high-temperature stretching, the filament cross-sections are distorted and weak spots are created in the cord which limit the tensions that can be applied to the cord without incurring excessive breakage. Such limited tensions greatly decrease the maximum yarn-tenacity attainable and limit cord strength.

SUMMARY OF THE INVENTION It is an object of this invention to provide a very hightenacity tire-yarn having a high degree of orientation of its crystalline domains characterized by a highly perfect molecular arrangement substantially free from defects such as regular chain-folding.

It is also an object to provide a novel process for the manufacture of these tire yarns.

The novel process of this invention comprises stretching a drawn continuous-filament polyester or polyamide yarn, previously twisted to less than 5 turns per inch and exhibiting an X-ray orientation angle of at least 10 degrees, at a stretching temperature of 200 to 260 0., exposing the yarn to the stretching temperature for 5 to 120 seconds, maintaining on the yarn during exposure to the stretching temperature a tension which is 75 to 98% of the maximum break-tension of the yarn measured at the stretching temperature, and cooling the stretched yarn below its second-order transition temperature under said constant tension.

Patented Feb. 23, 1971 The improved product of this invention is a continuous filament polyester or polyamide yarn with less than 5 turns per inch characterized by a tenacity in excess of 9.5 gm./denier; an X-ray orientation angle of less than 10 degrees; substantially undistorted uniform cross-sections of individual filaments along their lengths; and a minimum of regular chain-folding as indicated by X-ray long-period intensities.

The novel yarns of this invention possess a heretofore unattained level of orientation of crystalline domains and significantly higher tenacities than presently available in highly-drawn polyester or polyamide industrial yarns. The level of crystallinity, however, is abnormally low for such extreme exposures to heat. It is believed that the high tension on the yarn during heat-treatment retards crystallization during exposure to the high stretching-temperatures. Thus, the yarns have highly developed X-ray orientation without the excessive crystallization which results from high-temperature treatment at known lower tensions. The filaments of the multifilament yarns of this invention exhibit cross-sections of substantially constant shape throughout their lengths, in contrast to the deformed cross-sections resulting when highly twisted yarns or cords are similarly treated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of an apparatus arrangement for heating and stretching yarns at constant tension.

FIG. 2 is a graphical presentation of tenacity versus stretching tension for polyhexamethylene adipamide (nylon-66) yarns.

FIG. 3 presents graphically the variation in tenacity with stretching temperature for nylon-66.

FIG. 4 is a graphical display for nylon-66 of the dependence of modulus on time of exposure to the stretching temperature.

FIG. 5 is a graphical display for nylon-66 of the dependence of tenacity on time of exposure to the stretch ing temperature.

FIG. 6 presnts graphically the tenacity improvement for nylon-66 yarns resulting when care is taken to avoid thermal degradation.

FIG. 7 is a graphical presentation of the dependence of tenacity of polyethylene terephthalate yarns on the stretching tension employed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 represents a preferred process by which the novel yarns of this invention can be prepared. Yarn 10 is led from a supply package 12 and wrapped a plurality of times around driven variable-speed feed roll 14 and associated separator roll 16, passing therefrom through opening 18 into oven 20. Yarn 10 passes freely in multiple turns about rolls 22, 22 in oven 20 and then out through opening 24 where it successively passes over idler roll 26, over dancer roll 28, over idler roll 30, and around draw roll 32 and separator roll 34 on to a conventional windup (not shown) where it is packaged at substantially constant tension. The degree of stretch applied to yarn 10 is determined by the peripheral speeds of draw roll 32 and feed roll 14. Dancer roll 28 has coupled thereto a conventional control means 36 and a weight 38. The magnitude of the weight controls the stretching tension in oven 20. Control means 36 is coupled to variablespeed drive roll 14 to vary the speed thereof in accordance with vertical displacement of dancer roll 28, thereby maintaining constant tension and constant windup rate regardless of denier variations along the length of feedyarn 10. In the embodiment shown, rolls 22, 22' within oven 20 are circumferentially and helically grooved and free to turn independently. The oven is heated by a hot gas introducedthrough inlets 21 and removed through outlets 23. Temperature within oven 20 is maintained constant by conventional temperature sensing and control means (not shown). A commercial single-stage unit readily adaptable to the process of this invention is described by C. A. Litzler in Modern Textiles 35, p. 34 ff. (February 1954).

While the above represents a preferred process for producing the yarns of this invention, numerous alternatives within the scope of the invention are likewise suitable. Feed-yarn is preferably a highly drawn conventional multifilament tire yarn. Less highly drawn yarns are operable provided they are sufliciently crystalline for X-ray measurement of orientation angle. Known polyester and polyamide feed yarns have orientation angles greater than 10 degrees. The inclusion of rolls 22, 22 in oven enables shortening oven 20 for a given exposure time to its high temperature, but ovens free from internal rolls such that yarn 10 passes straight through without changes in direction are preferable where space allows. Any of several alternative means for maintaining constant yarn tension in the stretching and cooling zones can be employed, including well-known yarn tensiometers. Although the device is preferably coupled with automatic speed-control means for feed-roll 14 in order to maintain constant tension, many yarns 10 are available for which denier and stretching properties along their length are so nearly invariant that visually monitoring a tensiometer and manually controlling feed roll speed to maintain constant tension suffice.

The customary heating fluid for heating yarn 10 in oven 20 is hot air. Heater plates can, of course, be substituted. As is well-known, however, polymer degradation is quite rapid at the oven-temperatures of this invention. It is found that the high tensions employed in this invention increase the already high rate of degradation. Non-oxidizing gases are greatly preferred for heating yarn 10 in oven 20, e.g., steam or inert gases such as nitrogen. It is further preferable that yarn 10 comprise polymer containing an antioxidant, such as that described by Stamatoff in US. Pat. No. 2,705,227 and comprising copper in dissolved form, a halogen compound (e.g., KI), and (optionally) a phosphorous compound comprising inorganic phosphorous acids and alkali metal salts thereof. The high initial moduli of the yarns of this invention generally increase with increasing temperature, time of exposure, and tension in the heating and stretching zone when the three are within the ranges specified for this invention. The significant increase in tenacity provided by this invention is larger as more care is taken toavoid degradation of the yarn during exposure to the high temperatures.

Critical variables in the process of this invention are the temperature, time of exposure to the temperature, and tension on the yarn during both the exposure and the subsequent cooling. Stretching temperatures are in the range from 200 to 260 C. While the maximum temperature exceeds the normal melting point of unstressed crystalline polymer, the actual melting point increases with increasing yarn-tension. A temperature of about 274 C. is found to be the maximum at which nylon-66 yarn can be stretched without breakage. At constant exposure-time and tension within their respective operable ranges, tenacity of the resultant product increases to a maximum with increasing temperature and then decreases with continued increase in temperature. Preferred temperatures for operation according to this invention are in the range from 220 to 255 C.

Tension on the yarn during heating and stretching must be unprecedentedly high; i.e., from 75 to 98% of the maximum break-tension at the temperature of operation. There is a unique maximum break-tension for a yarn at each stretching temperature, decreasing with increasing temperature. The maximum break-tension is strongly influenced by smoothness and freedom from flaws of any roll-surfaces contacting the tensioned and heated yarn. If any flaws exist capable of lowering the maximum attainable tension, they will cause breakage of individual filaments and visible fuzziness of the yarn before it breaks. If the yarn breaks abruptly without any prior visible fuzziness, the rolls are sufiicie-ntly defect-free and the measured break-tension is the maximum attainable. Within the operable range of tensions specified, and at constant temperature and exposure time, tenacity of the yarn increases to a maximum and then decreases as tension varies from 75-98% of the maximum break-tension. The preferred tension is to 98% of maximum break-tension. Tensions employed nearly always exceed 3.0 gm./den., but operation at the highest temperatures in the operable range can require lowering the tension to below that absolute value.

Times of exposure to the stretching temperature are ordinarily within 5 to sec. The minimum time selected is a function of the heat-transfer conditions employed. Thus, if the yarn is more quickly brought to the temperature of the heating medium, relatively short treatment time is required to attain a given increase in yarn-tenacity. As with the other two variables, tenacity attained goes through a maximum with increasing exposure time at constant temperature and tension. Preferred exposure times are in the'range from 20 to 60 seconds.

After the yarn leaves the oven, it is necessary to cool it at the stretching tension sufiiciently to prevent loss of the high orientation introduced by stretching. Temperatures below the second-order transition temperature sufiice to prevent loss of orientation. As is well-known, the second-order transition temperature is dependent on the extents of crystallization and orientation of the polymer, and different methods of measurement yield different values. It is customary to cool a hot-drawn yarn by passage through ambient air. The times required for necessary cooling are well-known; ordinarily a yarntemperature below 80 C. is satisfactory for preventing loss of orientation. More rapid cooling methods may, of course, be employed.

Yarn 10, supplied to the process of this invention, is of very low twist, preferably untwisted. Depending on the mechanical equipment available, some twist may be necessary for satisfactory yarn processing. It is found that 5 turns/inch (2.0 turns/cm.) does not significantly limit the tenacity attainable. More highly twisted yarn or, in particularly, corded yarn, introduces many filament crossings which, during high-temperature stretching, seriously distort filament cross-sections and thereby greatly decrease the maximum yarn-tenacity attainable. Thus, the process of this invention wherein low-twist yarn is stretched to high tenacity and then corded yields cords of substantially higher tenacity than can be obtained if cording precedes stretching according to this invention.

Tenacities of polyester and polyamide yarns according to this invention are higher than heretofore commercially available, exceeding 9.5 g.p.d. Polyhexamethylene adipamide (nylon-66) tenacities in excess of 11.5 gm./ denier and polyethylene terephthalate (2GT) tenacities in excess of 10 gm./denier are readily attained.

Yarn tensile properties reported herein are determined using an Instron Tensile Tester, operated at 60% extension per minute with 10 inch (25.4 cm.) samples. All yarns are conditioned relaxed (in skein form) for 24 hours at 55% relative humidity and 75 F. (23.9 C.), before testing, which is also carried out at the same conditions.

The high tenacities of the crystalline polyester and polyamide yarns of this invention result from their unique structure on the molecular level. Thus, the high temperatures and tensions of the process produce yarns with higher orientation of the crystalline domains than known heretofore (i.e., orientation angles of less than 10 degrees) and with a minimum of chain-folding.

The technique for measurement of the orientation angle is shown in Figure 16 of Handbook of X-rays, chapter 21, Characterization of Polymers, 1967, McGraw-Hill Book Company, New York; and the fiat-plate wide angle X-ray diffraction patterns required for the measurement are obtained by the general procedures given in the same reference. Specifically, use is made of the pinhole collimator shown in .Figure 2 of the above chapter. The pinhole is 0.020 inch in diameter and the shelf width is only 0.004 inch since it is very important to use a small sample thickness in order to insure good parallel alignment of the filaments and an accurate measurement of the orientation angle. A 5.0 cm. sample-to-film distance is used with Kodak No-Screen film or its equivalent. After standard development, the film is densitometered to obtain the azimuthal intensity distribution of the diffraction arc, using a Leeds and Northrup Microphotometer with a 0.5 mm. height of slit or a Joyce-Loebl Microphotometer with 0.5 mm. height and 0.4 mm. width of slit (Model MK III CS with Polar Table MK III B, Joyce, Loebl & Co., Ltd., Gateshead-on-Tyne, England). The background on the full azimuthal tracing is chosen to be the average of the lowest intensities between the peaks. The smaller the orientation angle, the better is the alignment of crystalline domains with respect to the fiber axis. The reflection used for the reported measurements is the major equatorial difiraction having the smallest Bragg angle.

Calibrated techniques, different for each polymer, are required for determining the extent of crystallization in a given polymer sample, as is well known. For polyethylene terephthalate, a crystallinity index can be measured as shown in FIG. 7 of Handbook of X-Rays, chapter 21. For nylon-66, a crystal perfection index is obtained as described by P. F. Dismore and W. O. Statton in J. Polymer Sci., C13, pp. 133-148 (1966).

A unique and useful characteristic of the polyamide yarns of this invention is that, while crystalline orientation is unprecedentedly high and extent of crystallization is desirably lower than anticipated for the high temperatures of exposure, both resulting in new high levels of tenacity, a broad range of initial moduli for the yarns may be obtained. As shown in FIGS. 4 and 5 and described in Example IV, initial modulus increases with increasing temperature of exposure and with increasing time of exposure at a given temperature over a range in which the high tenacity remains substantially equivalent to the maximum attainable. Known drawing techniques, to produce higher tenacities, also increase initial modulus as a more or less single-valued function of tenacity. The process of this invention permits obtaining the new high level of tenacity of polyamides with or without a correspondingly great increase in modulus. Tire cords, for example, desirably have high moduli for overcoming the phenomenon known as flat-spotting, but maximization of modulus with increased tenacity is not necessarily desirable in other applications.

The yarns of this invention are strucutrally unique on the molecular level in a further respect, as confirmed by specific X-ray diffraction, nuclear magnetic resonance, and infrared measurements (described hereinafter). The exact interpretation of the results of these measurements has been a matter of some controversy in the literature, but recent research has indicated that the tests are related to the extent of regular chain-folding along molecular chains within a drawn yarn. For the purposes of this description, that interpretation is adopted, recognizing that modification of the interpretation may occur in future years. Chain-folding refers to the tendency for an extended polymer molecule to retract or rearrange by folding back on itself along its length when heated to high temperatures. Such folds are defects in the structure which do not support tensile loading. These inhomogeneities cause increased intensities of small-angle X-ray difiraction at points where the folds occur, additional motional freedom is provided since the folds eliminate some motional constraints, and more fiuidlike mobility results. The yarns of this invention are characterized as possessing low levels of regular chain-folding for treatment at such elevated temperatures.

Small-angle X-ray diffraction measurements by standard flat-plate photographic technique (using the Warhus camera as described in the above Handbook of X-Rays reference) provide a measure of chain-folding. In photographing the fibers, extreme care is taken to use the same amount of fiber in the X-ray beam for each exposure and to have the same exposure conditions for each determination. Long-period dilfraction streaks observed on the plate are typically (1) meridional spots, or (2) streaks across the meridian, or (3) streaks connecting a 4-point off-meridional diagram, whichever may exist. Quantitative measurement of diffraction intensity is obtained using a densitometer on the flat plate, and the X-ray long-period intensity is reported as maximum optical density along a given diffraction streak (i.e., a trace is obtained parallel to the equator through the 4- point streak). The lower this optical density, the less is the degree of regular chain-folding.

Example I TABLE I.COMPARISON OF FEED YARN WITH HOT STRETCHED YARN Hot- Feed stretched yarn yarn Test code 95125 Tenacity (gpd 9.6 Elongation (percent). 7. 3 Initial modulus (g.p.d.) 151 Orientation angle (degre 7. 5 NMR peak ratio at 200 C 1. 5 Applied stretch, percent 23. 1 X-ray long-period intensity (optical density units) 0 44 0. 36

Example 11 This example shows the necessity for achieving a high stretch-tension in order to produce the yam-structure of this invention.

High-tenacity drawn 2G-T yarn is hot-stretched in an oven as shown in FIG. 1. In order to prevent thermal degradation, a nitrogen atmosphere is maintained in the oven. Exposure time in the oven is 60 seconds. Table II shows that, at a given temperature and exposure time, the desired increase in tenacity is not obtained at low yarn tension.

TABLE II.EFFECT OF YARN-TENSION DURING HOT- STRETGHIN G Drawn Low-tenieed sion hot- This inyarn stretched vention Test code 32-18 32-9 32-10 Stretch temp. C.) 220 220 Stretch tension (g.p.d.) 2. 1 3. 0 Applied stretch (percent) 10 20 Net stretch (percent) 4. 1 14. 1 Tenacity (g.p.d.) 8. 5 8. 7 9. 8 Elongation (percent) 14.1 9. l 8. 4 Initial modulus (g.p.d.) 101 116 126 Orientation angle 12. 4 11. 4 7. 9 NMR ratio at 200 C 2.0 1. 7 1. 4 Infrared folding coefficient 0.22 0. 22 0. 19 X-ray long-period intensity (optical density units) 0. 38 0. 47 0. 33

Example III A highly drawn nylon-66 tire yarn of 1072 denier is hot-stretched according to this invention, heating in the hot-chest being by steam at 480 F. (249 C.). The feed yarn contains by weight, about 0.02% copper acetate and 0.02% potassium iodide as antioxidants. The yarn is drawn straight through the hot-chest (i.e., rolls 22 and 22 of FIG. 1 are not used), and residence time in the hotchest is 30 seconds. The tension applied to the yarn during hot-stretching is 8.65 pounds (92% of the maximum break-tension at the temperature of treatment). Based on denier of the feed yarn, this tension is 3.66 g.p.d. Conventional cooling of the yarn in air occurs while tension is maintained. Stretch applied to the yarn between feed-roll 14 and draw-roll 32 is about 46.6%, and the net retained stretch of the yarn is about 35.6%. Final denier is 770. Table III compares the two yarns [values in A/B form are at 23.9 C. (A) and 75 C. (B)].

TABLE III.COMPARISON OF FEED AND I'IOT- STRETCHED NYLON YARN Feed This yarn example Tenacity (g.p.d.) .84/7. 32 11.87/10.18 Elongation (percent) 19. 6/23. 7 11. 9/13. 9 Initial modulus (g.p.d.) 40. 2/25. 3 71. 5/39. 1 Orientation angle (degrees) 10. 9 8. 1 Crystal perfection index 91 78 With all conditions constant except for the heat transfer medium for heating tensioned nylon yarn, the use of steam yields slightly higher maximum tenacity than does heated nitrogen, both yielding tenacities higher than obtainable with heated air.

Example IV 8 yarns (S and T) are seen to contain 1X antioxidant, i.e., only one-half the above concentration. The control yarn for these two tests is a highly drawn yarn very similar in properties to those shown for control yarn in Table V, but nominally of only 840 total denier and 140 continuous filaments.

As in Example II, the high-temperature stretching of yarn in this example is accomplished by passing the yarn straight through the oven, no supporting rolls being present in the oven. Most of the samples of Tables IV and V illustrate the use of nitrogen as heat-transfer medium. To accomplish this economically, this example utilizes a metal pipe extending through the oven so as to enclose the yarn along its path with no yarn-to-pipe contact. Copper tubing for nitrogen supply is tightly coiled outside the pipe along about one-half of its length, discharging nitrogen into the pipe at its lengthwise mid-point. Temperature-controlled hot air passing through the oven preheats both the nitrogen in its copper tubing and the straight pipe along its whole length. Thus, the heated yarn is contacted only by nitrogen at the specified temperature.

A very large number of tests is performed in support of the advantages of this invention. To the extent that each of these tests applies to the comparisons made in FIGS. 2 through 6, all tests are represented by points on those figures. Tables IV and V completely characterize tests selected to be representative of the whole range of temperature, tension, time, heat-transfer medium, and antioxidant-concentration variables, the codes of the tests identifying the selected points in FIGS. 2 through 6. In Table V, heat-transfer media employed are designated N for nitrogen and A for air. Temperatures shown are those to which the air circulated through the oven is heated and controlled. Using known heat-transfer data and relationships, it is computed that approximately 11 sec. of exposure to nitrogen at 470 F. (243.3 C.) is required for the yarn to essentially attain the same temperature. In about 4.8 sec., the yarn-temperature increases to 392 F. (200 C.).

Averaged maximum break-tensions are obtained at each of a range of constant temperatures using rolls so smooth that yarn failure is abrupt with no prior failure of individual filaments. Table IV shows for each test the total tension on the yarn within the oven and its percentage of the maximum break-tension.

TABLE IV.NYLON-GG, PROCESSING CONDITIONS FOR EXAMPLE IV Percent Transfer Temperature, Tension, break Time Antimedinn u. F.) lb. (kg) tension (sec.) oxidant 215. 6(420) 9. 69 (4. 96 30 2X 215. 6(420) 9. 49 (4. 30) 94 30 2X 215. 6(420) 8. 46 (3. 84) 90 30 2X 215. 6(420) 9. 29 (4. 21) 92 30 2X 257. 2(495) 7. 72 (3. 92 30 2X 215. 6(42()) 5. 22 (2. 37) 50 30 2X 215. 6(420) 7. 31(3. 32) 70 30 2X 257. 2(495) 7. 81) 3. 54) 92 120 EX 237. 8(460) 9. 06(4. 11) 98 30 2X 257. 2(495) 6. 62 (3. 00) 92 2X 257. 2(495) 7. 69(3. 49) 92 30 2X 237. 8(460) 7. 86(3. 57) 85 30 2X 237. 8(460) 7. 40(3. 36) 80 30 2X 237. 8(460) 8. 32 (3. 77) 90 30 2X 237. 8(460) 6. 47 (2. 93) 30 2X 237. 8(460) 4. 62 (2. 10) 53 30 2X 237. 8(460) 9. 20 (4. 17) 92 30 2X 237. 8(460) 7. 86(3. 57) 92 5 1X 237. 8(460) 7. 30 (3. 31) 92 15 1X 257. 2(495) 6. 9 3. 13) 94 30 2X 257. 2(495) 7. 87 (3. 57) 92 20 2X 257 2(495) 8. 18(3. 71) 92 10 2X 237. 8(460) 8. 14 (3. 69) 92 30 2X 257. 2 (495) 6. 44 (2. 92) 70 30 2X 257. 2(495) 7. 26(3. 29) 30 2X long period Crystal intensity perfection X-ray optical index density) Code:

angle values of X-ray long-period intensity indicate reduced regular chain-folding.

TABLE VI.LOWE RING OF CHAIN-FOLDING WITH INC REASED TENSION [Temperature, 237.8 0.; Treatment time, 30 sec.]

X-ray Percent longbreakperiod Tenacity Orientation tension intensity (g.p.d.)

Code:

The times shown are the number of seconds of exposure to the heat-transfer medium during hot-stretching, times from to 120 seconds being included.

FIG. 2 presents the variation in attainable tenacity as a function of the percentage of maximum break-tension employed, curves for three stretching temperatures within the operable temperature range being shown. The

melting point for unstressed nylon-66 is about 509 F. (265 C.). The stretching temperatures shown are, re-

spectively, below, within, and just above the preferred range. Bars 50, 50', 50 intersecting these curves show approximately the points where orientation angle changes from 10 or more degrees to less than 10 degrees.

FIG. 3 shows a variation in tenacity obtained as a function of treatment-temperature, all other processing variables held constant within the preferred range. Max- EXAMPLE V This example illustrates the process and product advantages of this invention as applied to yarns of polyimum tenacities are shown to result within the preferred temperature range of 220 to 255 C.

FIGS. 4 and 5 are graphs showing tenacity and modethylene terephthalate (2GT). The control yarn used ulus as a function of treatment-time. Again it is shown as starting material in these tests is a high-tenacity polythat highest tenacities result within the preferred teme ter ti e yarn composed of 192 continuous filaments curves it is seen providing a total denier of 1000. A twist of 3 turns/inch that, for each temperature, a broad range of treatmentperature range. From the tenacity (1.2 turns/cm.) is applied before stretching. Table VII times produces high tenacities not significantly different shows the processing conditions used as described in Exin absolute values; but the modulus curves show moduample IV; and Table VIII lists the properties of the yarn lus increasing continuously with increasing treatment products. times.

FIG. 6 illustrates the property improvement for lowtwist 2G-T yarn processed according to this invention, i.e., at tensions approaching the break-tension for each 5 temperature. The products which were stretched under a tension of at least 75% of the break-tension exhibit both In FIG. 6, highest tenacities are seen to result using nitrogen as heat-transfer medium and polymer with 2X antioxidant concentration (circles). Reducing antioxi- 5 dant concentration to IX (triangles) or using air for heattransfer (squares) is seen to lower the attainable tenacities. Table VI below rearranges data from Tables IV and V to show the relationship of X-ray long-period intensity to tenacity and the percentage of break-tension. Low

TABLE VIII.POLYETHYLENE TEREPI-ITHALATE, PROPERTIES OF THE YARNS OF EXAMPLE V Orien- Tenaelty Elongation, Modulus tation Denier (g.p.d.) percent (g.p.d.) angle A test not represented in Tables VII and VIII further illustrates the requirement for high tensions during stretching at elevated temperatures. The control yarn is identical to the above. For the control, starting tenacity is 8.64 gm./den. and the X-ray long-period peak-intensity is 0.38 optical density units. The control is then heated as described herein at 220 C. under constant tensions corresponding to: (1) 20% shrinkage, (2) constant length, (3) 10% stretch, and (4) stretch. The X-ray long-period intensities are, respectively, 0.51, 0.50, 0.47, and 0.33. It is thus shown that less chain-folding than present in the control results only when 20% stretch is applied. At 10% stretch, tenacity developed is only 8.73 gm./den., i.e., about equal to that of the control. At 20% stretch, tenacity is 9.76 gm./den., a substantial improvement.

Polyester yarns advantageously treated according to the process of this invention are of those polyester polymers of fiber-forming viscosities which contain carbonylo-xy linking radicals as an integral part of the polymer chain. Examples of suitable, crystallizable, linear-condensation polyesters include polyethylene terephthalate, polyethylene terephthalate/ isophthalate (85/15), polyethylene terephthalate/S-sodium sulfo)isophthalate (97/3), poly(p-hexahydroxylylene terephthalate), polyhydroxypivalic acid, poly(decahydronaphthalene-2,6-dimethylene 4,4 bibenzoate), polyethylene 2,6- or 2,7-naphthalenedicarboxylate, and poly- (bicyclohexyl-4,4'-dimethylene-4,4 bibenzoate), as well as many others. Preferably the polyester is a linear glycol terephthalate polyester. By this is meant a linear condensation polyester derived from a glycol and an organic acid in which the glycol component is comprised substantially of a dihydroxy compound of a divalent saturated hydrocarbon radical containing from 2 to 10 carbon atoms and the acid component is at least about 75 mol-percent terephthalic acid.

Polyamides suitable for treatment as herein described are characterized by recurring amino radicals as an integral part of the polymer chain. The amido radicals are linked by divalent organic radicals which may be aliphatic, cycloalpihatic or aromatic, or mixtures of the above. Typical polyamides include poly(hexamethylene adipamide) referred to herein as nylon-66, polycaprolactam, poly(hexamethylene sebacamide), polyaminoundecanoamide, poly(hexamethylene isophthalamide), poly(2- methyl hexamethylene terephthalamide), poly(metaxylylene adipamide), poly(para-xylylene sebacamide), poly(octamethylene oxalarnide), and the polyamide from 12 bis(4-amino-cyclohexyl) methane and aliphatic acids such as dodecanedioic acid.

The hot-stretching process of this invention, wherein the constant tension applied to a crystalline low-twist yarn during stretching is from -98% of the break-tension and wherein the stretched yarn is cooled under the constant tension sufficiently to retain its increased molecular orientation, yields yarns of higher tenacity than heretofore attainable from a given crystalline polymer. The moduli of the yarn-products, moreover, are usually higher than previously attainable. The yarn-products may be twisted, plied, corded, dipped, hot-stretched, and otherwise treated according to conventional procedures for manufacture of cords for tires and industrial uses.

Broad line nuclear magnetic resonance (N.M.R.) as described in Segment Mobility in Fibers as Shown by High Temperature N.M.R., W. O. Statton, American Dyestuif Reporter, 54, 26-33, April 1965 and an infrared technique as described by J. L. Koenig and M. J. Hannon in J. Macromol, Sci. (Phys), B1(1), 119-145 (1967), are other techniques for measuring chain folding. When each of these techniques has been applied to yarns of this invention, the resultant analysis shows such yarns to have a degree of chain folding lower than heretofore observed in yarns exposed to such high temperatures.

What is claimed is:

'1. A drawn continuous multifilament yarn selected from the group consisting of polyesters and polyamides having less than 5 turns per inch of twist, the filaments of said yarn being substantially uniform in cross section throughout their length, said filaments being characterized by crystalline domains having an average orientation angle of less than 10 degrees.

2. A drawn continuous multifilament polyester yarn having less than 5 turns per inch of twist, the filaments of said yarn being substantially uniform in cross section throughout their length, said filaments being characterized by crystalline domains having an orientation angle of less than about 8.5 degrees and an X-ray long-period intensity of less than 0.4 optical density units.

3. The yarn of claim 2, said yarn being constituted essentially of polyethylene terephthalate.

4. A drawn continuous multifilament polyamide yarn having less than 5 turns per inch of twist, the filaments of said yarn being substantially uniform in cross section throughout their length, said filaments being characterized by crystalline domains having an orientation angle of less than about 10 degrees and an X-ray long-period intensity of less than about 0.2 optical density units.

5. The yarn of claim 4, said yarn being constituted essentially of polyhexamethylene adipamide.

References Cited UNITED STATES PATENTS 2,807,863 10/ 1957 Schenker I 28-72 2,955,344 10/1960 Prettyman 28-72 3,216,187 11/1965 Chantry et al. 57--140 3,343,363 9/1967 Stow, Jr., et al. 57140 3,409,958 11/ 1968 Bucher et al. 2871.3 3,413,797 12/1968 Chapman 5714O FOREIGN PATENTS 1,081,089 8/1967 Great Britain 57-140 JOHN PETRAKES, Primary Examiner US. Cl. X.R. 

